The invention relates to a transmission system for the transmission of signal elements of an information signal which elements are spaced at fixed clock intervals, by means of a pulse code comprising a plurality of pulses, and to transmitters and receivers to be used in said system. The transmitter is provided with a pulse code modulation device, which, before the transmission to the receiver, converts the signal elements applied to a input of said device into the pulse code comprising a plurality of pulses, while the receiver is provided with a pulse code demodulation device, which reproduces the transmitted information signal from the pulse code.
In pulse code modulation the amplitude quantization generally gives rise to deviations between the information signal reproduced by the receiver and the original information signal, which deviations cause so-called quantization noise and adversely affect the reproduction quality. If the number of pulses in the pulse code is increased, the deviations between the reproduced information signal and the original information signal decrease and the reproduction accuracy consequently increases, so that for a signal transmission of satisfactory reproduction quality a comparatively large number of pulses in the pulse code and thus large transmission bandwidths are needed.
In order to reduce the influence of the quantization noise and thus realize a satisfactory reproduction quality using a pulse code with a reduced number of pulses, it is advantageous to employ instantaneous dynamic control, which is characte zed by a non-linear amplitude quantization in the pulse code which is used, in particular so as to effect a substantial reduction of the transmission bandwidth as is inter alia required for image telephony (videophone) transmission. For example, in a known videophone transmission system adapted for the transmission of a videophone signal of 1 MHz bandwidth, employing differential pulse code transmission, a differential pulse code, consisting of only three pulses per signal element situated in 0.5 .mu.sec clock intervals, was used in order to realize the minimum transmission bandwidth of 3 MHz. However, despite the use of the non-linear amplitude quantization, it was found that at this transmission bandwidth, which had been reduced to the minimum value, the reproduction quality in the transmission system was still found to be particularly unfavourable.